U.S. patent application number 14/552637 was filed with the patent office on 2015-06-04 for conductive paste composition and semiconductor devices made therewith.
The applicant listed for this patent is E I DU PONT DE NEMOURS AND COMPANY. Invention is credited to CARMINE TORARDI, PAUL DOUGLAS VERNOOY.
Application Number | 20150155403 14/552637 |
Document ID | / |
Family ID | 53266018 |
Filed Date | 2015-06-04 |
United States Patent
Application |
20150155403 |
Kind Code |
A1 |
TORARDI; CARMINE ; et
al. |
June 4, 2015 |
CONDUCTIVE PASTE COMPOSITION AND SEMICONDUCTOR DEVICES MADE
THEREWITH
Abstract
A conductive paste composition contains a source of an
electrically conductive metal, an alkaline-earth-metal boron
bismuth oxide, and an organic vehicle. An article such as a
high-efficiency photovoltaic cell is formed by a process of
deposition of the paste composition on a semiconductor substrate
(e.g., by screen printing) and firing the paste to remove the
organic vehicle and sinter the metal and alkaline-earth-metal boron
bismuth oxide.
Inventors: |
TORARDI; CARMINE;
(Wilmington, DE) ; VERNOOY; PAUL DOUGLAS;
(Hockessin, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
E I DU PONT DE NEMOURS AND COMPANY |
Wilmington |
DE |
US |
|
|
Family ID: |
53266018 |
Appl. No.: |
14/552637 |
Filed: |
November 25, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61911026 |
Dec 3, 2013 |
|
|
|
Current U.S.
Class: |
136/256 ;
252/514; 427/126.3; 427/126.5; 428/446; 428/697 |
Current CPC
Class: |
H01L 31/022425 20130101;
C09D 11/52 20130101; C09D 1/00 20130101; C09D 5/24 20130101; C09D
7/40 20180101; C09D 11/037 20130101; C09D 11/033 20130101; Y02E
10/50 20130101 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; C09D 5/24 20060101 C09D005/24 |
Claims
1. A paste composition comprising: (a) a source of electrically
conductive metal; (b) an alkaline-earth-metal boron bismuth oxide;
and (c) an organic vehicle, in which the source of electrically
conductive metal and the oxide are dispersed.
2. The paste composition of claim 1, comprising 0.5 to 10 weight %
of the alkaline-earth-metal boron bismuth oxide.
3. The paste composition of claim 1, wherein alkaline-earth metal,
boron, and bismuth cations comprise 75 to 95 cation % of the
alkaline-earth-metal boron bismuth oxide.
4. The paste composition of claim 1, wherein the
alkaline-earth-metal boron bismuth oxide comprises: 10 to 40 cation
% of an alkaline earth metal selected from the group of Mg, Ca, Ba,
Sr, and mixtures thereof; 14 to 65 cation % of B; and 10 to 60
cation % of Bi, plus incidental impurities.
5. The paste composition of claim 4, wherein the
alkaline-earth-metal boron bismuth oxide further comprises at least
one oxide selected from the group consisting of oxides of Al, Li,
Na, K, Rb, Cs, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Si, Mo,
W, Hf, Ag, Ga, Ge, In, Sn, Sb, Se, Ru, P, Y, La and the other
lanthanide elements, and mixtures thereof.
6. The paste composition of claim 5, wherein the
alkaline-earth-metal boron bismuth oxide further comprises at least
one oxide selected from the group consisting of oxides of Li, Na,
Si, P, Zn, and Ti.
7. The paste composition of claim 6, wherein the
alkaline-earth-metal boron bismuth oxide further comprises: 0 to 15
cation % of Li; 0 to 15 cation % of Na; 0 to 15 cation % of Si; 0
to 15 cation % of P; 0 to 20 cation % of Zn; and 0 to 20 cation %
of Ti, plus incidental impurities.
8. The paste composition of claim 1, wherein up to 10 anion percent
of the oxygen anions of the alkaline-earth-metal boron bismuth
oxide are replaced by halogen anions.
9. The paste composition of claim 1, wherein the source of the
electrically conductive metal is an electrically conductive metal
powder.
10. The paste composition of claim 1, wherein the electrically
conductive metal comprises Ag.
11. The paste composition of claim 10, wherein the Ag comprises 85
to 99.5 wt. % of the solids in the composition.
12. The paste composition of claim 1, wherein the paste composition
is lead-free.
13. The paste composition of claim 1, further comprising an oxide
additive that is an oxide of Al, Li, Na, K, Rb, Cs, Ti, V, Cr, Mn,
Fe, Co, Ni, Cu, Zn, Zr, Nb, Si, Mo, W, Hf, Ag, Ga, Ge, In, Sn, Sb,
Se, Ru, Bi, Ba, Ca, Sr, Mg, B, P, Y, La or the other lanthanide
elements, or mixtures thereof, or a compound of one or more of the
above elements which form an oxide upon firing.
14. A process for forming an electrically conductive structure on a
substrate, the process comprising: (a) providing a substrate having
a first major surface; (b) applying a paste composition onto a
preselected portion of the first major surface, wherein the paste
composition comprises in admixture: i) a source of electrically
conductive metal, ii) an alkaline-earth-metal boron bismuth oxide,
and iii) an organic vehicle, in which the source of electrically
conductive metal and the oxide are dispersed; and (c) firing the
substrate and paste composition thereon, whereby the electrically
conductive structure is formed on the substrate.
15. The process of claim 14, wherein the source of electrically
conductive metal is silver powder.
16. The process of claim 14, wherein the substrate comprises an
insulating layer present on at least the first major surface and
comprising at least one layer comprised of aluminum oxide, titanium
oxide, silicon nitride, SiN.sub.x:H, silicon oxide, or silicon
oxide/titanium oxide, the paste composition is applied onto the
insulating layer of the first major surface, and the insulating
layer is penetrated and the electrically conductive metal is
sintered during the firing, whereby an electrical contact is formed
between the electrically conductive metal and the substrate.
17. An article comprising a substrate and an electrically
conductive structure thereon, the article having been formed by the
process of claim 14.
18. The article of claim 17, wherein the substrate is a silicon
wafer.
19. The article of claim 17, wherein the article comprises a
semiconductor device.
20. The article of claim 19, wherein the article comprises a
photovoltaic cell.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims benefit of U.S. Provisional Patent
Application Ser. No. 61/911,026, filed Dec. 3, 2013 and entitled
"Conductive Paste Composition and Semiconductor Devices Made
Therewith," which is incorporated herein in its entirety by
reference thereto.
FIELD OF THE INVENTION
[0002] The present invention relates to a conductive paste
composition that is useful in the construction of a variety of
electrical and electronic devices, and more particularly to a paste
composition useful in creating conductive structures, including
front-side electrodes for photovoltaic devices.
TECHNICAL BACKGROUND OF THE INVENTION
[0003] A conventional photovoltaic cell incorporates a
semiconductor structure with a junction, such as a p-n junction
formed with an n-type semiconductor and a p-type semiconductor. For
the typical p-base configuration, a negative electrode is located
on the side of the cell that is to be exposed to a light source
(the "front" side, which in the case of a solar cell is the side
exposed to sunlight), and a positive electrode is located on the
other side of the cell (the "back" side). Radiation of an
appropriate wavelength, such as sunlight, falling on the p-n
junction serves as a source of external energy that generates
electron-hole pair charge carriers. These electron-hole pair charge
carriers migrate in the electric field generated by the p-n
junction and are collected by electrodes on respective surfaces of
the semiconductor. The cell is thus adapted to supply electric
current to an electrical load connected to the electrodes, thereby
providing electrical energy converted from the incoming solar
energy that can do useful work. Solar-powered photovoltaic systems
are considered to be environmentally beneficial in that they reduce
the need for fossil fuels used in conventional electric power
plants.
[0004] Industrial photovoltaic cells are commonly provided in the
form of a structure, such as one based on a doped crystalline
silicon wafer, that has been metalized, i.e., provided with
electrodes in the form of electrically conductive metal contacts
through which the generated current can flow to an external
electrical circuit load. Most commonly, these electrodes are
provided on opposite sides of a generally planar cell structure.
Conventionally, they are produced by applying suitable conductive
metal pastes to the respective surfaces of the semiconductor body
and thereafter firing the pastes.
[0005] Photovoltaic cells are commonly fabricated with an
insulating layer on their front side to afford an anti-reflective
property that maximizes the utilization of incident light. However,
in this configuration, the insulating layer normally must be
removed to allow an overlaid front-side electrode to make contact
with the underlying semiconductor surface. The front-side
conductive metal paste typically includes a glass frit and a
conductive species (e.g., silver particles) carried in an organic
medium that functions as a vehicle for printing. The electrode may
be formed by depositing the paste composition in a suitable pattern
(for instance, by screen printing) and thereafter firing the paste
composition and substrate to dissolve or otherwise penetrate the
insulating anti-reflective layer and sinter the metal powder, such
that an electrical connection with the semiconductor structure is
formed.
[0006] The ability of the paste composition to penetrate or etch
through the anti-reflective layer and form a strong adhesive bond
with the substrate upon firing is highly dependent on the
composition of the conductive paste and the firing conditions. Key
measures of photovoltaic cell electrical performance, such as
efficiency, are also influenced by the quality of the electrical
contact made between the fired conductive paste and the
substrate.
[0007] Although various methods and compositions useful in forming
devices such as photovoltaic cells are known, there nevertheless
remains a need for compositions that permit fabrication of
patterned conductive structures that provide improved overall
device electrical performance and that facilitate the efficient
manufacture of such devices.
SUMMARY OF THE INVENTION
[0008] An embodiment of the invention relates to a paste
composition comprising:
[0009] (a) a source of electrically conductive metal;
[0010] (b) an alkaline-earth-metal boron bismuth oxide; and
[0011] (c) an organic vehicle, in which the source of electrically
conductive metal and the oxide are dispersed.
[0012] In certain embodiments, the alkaline-earth-metal boron
bismuth oxide further comprises an oxide of an additional cation,
including, without limitation, an oxide of any one or more of Al,
Li, Na, K, Rb, Cs, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Si,
Mo, W, Hf, Ag, Ga, Ge, In, Sn, Sb, Se, Ru, P, Y, La and the other
lanthanide elements, and mixtures thereof, or substances that form
such oxides during heating. Other embodiments of the paste
composition further comprise one or more discrete oxide additives,
including, without limitation, oxides of Al, Li, Na, K, Rb, Cs, Ti,
V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Si, Mo, W, Hf, Ag, Ga, Ge,
In, Sn, Sb, Se, Ru, Bi, Ba, Ca, Sr, Mg, B, P, Y, La or the other
lanthanide elements, or mixtures thereof, or a compound of one or
more of the above elements which form an oxide upon firing.
[0013] Another aspect provides a process for forming an
electrically conductive structure on a substrate, the process
comprising: [0014] (a) providing a substrate having a first major
surface; [0015] (b) applying a paste composition onto a preselected
portion of the first major surface, wherein the paste composition
comprises in admixture: [0016] i) a source of electrically
conductive metal, [0017] ii) an alkaline-earth-metal boron bismuth
oxide, and [0018] iii) an organic vehicle, in which the source of
electrically conductive metal and the oxide are dispersed; and
[0019] (c) firing the substrate and paste composition thereon,
whereby the electrically conductive structure is formed on the
substrate.
[0020] In a further implementation, the substrate includes an
anti-reflective layer on its surface, and the firing results in the
paste at least partially etching through the anti-reflective layer,
such that electrical contact between the conductive structure and
the substrate is established.
[0021] Further, there is provided an article comprising a substrate
and an electrically conductive structure thereon, the article
having been formed by the foregoing process. Representative
articles of this type include a semiconductor device and a
photovoltaic cell. In an embodiment, the substrate comprises a
silicon wafer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The invention will be more fully understood and further
advantages will become apparent when reference is made to the
following detailed description of the preferred embodiments of the
invention and the accompanying drawings, wherein like reference
numerals denote similar elements throughout the several views and
in which:
[0023] FIGS. 1A-1F depict successive steps of a process by which a
semiconductor device may be fabricated. The device in turn may be
incorporated into a photovoltaic cell. Reference numerals as used
in FIGS. 1A-1F include the following: [0024] 10: p-type substrate
[0025] 12: first major surface (front side) of substrate 10 [0026]
14: second major surface (back side) of substrate 10 [0027] 20:
n-type diffusion layer [0028] 30: insulating layer [0029] 40: p+
layer [0030] 60: aluminum paste formed on back side [0031] 61:
aluminum back electrode (obtained by firing back-side aluminum
paste) [0032] 70: silver or silver/aluminum paste formed on back
side [0033] 71: silver or silver/aluminum back electrode (obtained
by firing back-side paste) [0034] 500: conductive paste formed on
front side according to the invention [0035] 501: conductive front
electrode according to the invention (formed by firing front-side
conductive paste)
DETAILED DESCRIPTION OF THE INVENTION
[0036] The present invention addresses the need for a process to
manufacture high performance semiconductor devices having
mechanically robust, high conductivity electrodes. The conductive
paste composition provided herein is beneficially employed in the
fabrication of front-side electrodes of photovoltaic devices.
Ideally, a paste composition promotes the formation of a front-side
metallization that: (a) adheres strongly to the underlying
semiconductor substrate; and (b) provides a relatively low
resistance contact with the substrate. Suitable paste compositions
are believed to aid in etching surface insulating layers often
employed in semiconductor structures such as photovoltaic cells to
allow contact between the conductive electrodes and the underlying
semiconductor.
[0037] In an aspect, this invention provides a paste composition
that comprises: a functional conductive component, such as a source
of electrically conductive metal; an alkaline-earth-metal boron
bismuth oxide; an optional discrete inorganic additive; and an
organic vehicle. Certain embodiments involve a photovoltaic cell
that includes a conductive structure made with the present paste
composition. Such cells may provide any combination of one or more
of high photovoltaic conversion efficiency, high fill factor, and
low series resistance.
[0038] In various embodiments, the present paste composition may
comprise, in admixture, an inorganic solids portion comprising (a)
about 75% to about 99.5% by weight, or about 90 to about 99% by
weight, or about 95 to about 99% by weight, of a source of an
electrically conductive metal; (b) about 0.5% to about 15% by
weight, or about 0.5% to about 8% by weight, or about 2% to about
8% by weight, or about 0.5 to about 5% by weight, or about 1 to
about 3% by weight, of an alkaline-earth-metal boron bismuth oxide
material, wherein the above stated contents of constituents (a) and
(b) are based on the total weight of all the constituents of the
inorganic solids portion of the composition, apart from the organic
medium.
[0039] As further described below, the paste composition further
comprises an organic vehicle, which acts as a carrier for the
inorganic constituents, which are dispersed therein. The paste
composition may include still additional components such as
surfactants, thickeners, thixotropes, and binders.
[0040] Typically, electrodes and other conductive traces are
provided by screen printing the paste composition onto a substrate,
although other forms of printing, such as plating, extrusion,
inkjet, shaped or multiple printing, or ribbons may also be used.
After deposition, the composition, which typically comprises a
conductive metal powder (e.g., Ag), a frit, and optional inorganic
additives in an organic carrier, is fired at an elevated
temperature.
[0041] The composition also can be used to form conductive traces,
such as those employed in a semiconductor module that is to be
incorporated into an electrical or electronic device. As would be
recognized by a skilled artisan, the paste composition described
herein can be termed "conductive," meaning that the composition can
be formed into a structure and thereafter processed to exhibit an
electrical conductivity sufficient for conducting electrical
current between devices or circuitry connected thereto.
I. Inorganic Components
[0042] An embodiment of the present invention relates to a paste
composition, which may include: an inorganic solids portion
comprising a functional material providing electrical conductivity,
an alkaline-earth-metal boron bismuth oxide fusible material; and
an organic vehicle in which the inorganic solids are dispersed. The
paste composition may further include additional components such as
surfactants, thickeners, thixotropes, and binders.
A. Electrically Conductive Metal
[0043] The present paste composition includes a source of an
electrically conductive metal. Exemplary metals include without
limitation silver, gold, copper, nickel, palladium, platinum,
aluminum, and alloys and mixtures thereof. Silver is preferred for
its processability and high conductivity. However, a composition
including at least some non-precious metal may be used to reduce
cost.
[0044] The conductive metal may be incorporated directly in the
present paste composition as a metal powder. In another embodiment,
a mixture of two or more such metals is directly incorporated.
Alternatively, the metal is supplied by a metal oxide or salt that
decomposes upon exposure to the heat of firing to form the metal.
As used herein, the term "silver" is to be understood as referring
to elemental silver metal, alloys of silver, and mixtures thereof,
and may further include silver derived from silver oxide (Ag.sub.2O
or AgO) or silver salts such as AgCl, AgNO.sub.3, AgOOCCH.sub.3
(silver acetate), AgOOCF.sub.3 (silver trifluoroacetate),
Ag.sub.3PO.sub.4 (silver orthophosphate), or mixtures thereof. Any
other form of conductive metal compatible with the other components
of the paste composition also may be used.
[0045] Electrically conductive metal powder used in the present
paste composition may be supplied as finely divided particles
having any one or more of the following morphologies: a powder
form, a flake form, a spherical form, a rod form, a granular form,
a nodular form, a crystalline form, other irregular forms, or
mixtures thereof. The electrically conductive metal or source
thereof may also be provided in a colloidal suspension, in which
case the colloidal carrier would not be included in any calculation
of weight percentages of the solids of which the colloidal material
is part.
[0046] The particle size of the metal is not subject to any
particular limitation. As used herein, "particle size" is intended
to refer to "median particle size" or d.sub.50, by which is meant
the 50% volume distribution size. The distribution may also be
characterized by d.sub.90, meaning that 90% by volume of the
particles are smaller than d.sub.90. Volume distribution size may
be determined by a number of methods understood by one of skill in
the art, including but not limited to laser diffraction and
dispersion methods employed by a Microtrac particle size analyzer
(Montgomeryville, Pa.). Laser light scattering, e.g., using a model
LA-910 particle size analyzer available commercially from Horiba
Instruments Inc. (Irvine, Calif.), may also be used. In various
embodiments, the median particle size is greater than 0.2 .mu.m and
less than 10 .mu.m, or the median particle size is greater than 0.4
.mu.m and less than 5 .mu.m, as measured using the Horiba LA-910
analyzer.
[0047] The electrically conductive metal may comprise any of a
variety of percentages of the composition of the paste composition.
To attain high conductivity in a finished conductive structure, it
is generally preferable to have the concentration of the
electrically conductive metal be as high as possible while
maintaining other required characteristics of the paste composition
that relate to either processing or final use. In an embodiment,
the silver or other electrically conductive metal may comprise
about 75% to about 99.5% by weight, or about 85 to about 99.5% by
weight, or about 95 to about 99% by weight, of the inorganic solid
components of the paste composition. In another embodiment, the
solids portion of the paste composition may include about 80 to
about 90 wt. % silver particles and about 1 to about 9 wt. % silver
flakes. In an embodiment, the solids portion of the paste
composition may include about 70 to about 90 wt. % silver particles
and about 1 to about 9 wt. % silver flakes. In another embodiment,
the solids portion of the paste composition may include about 70 to
about 90 wt. % silver flakes and about 1 to about 9 wt. % of
colloidal silver. In a further embodiment, the solids portion of
the paste composition may include about 60 to about 90 wt. % of
silver particles or silver flakes and about 0.1 to about 20 wt. %
of colloidal silver.
[0048] The electrically conductive metal used herein, particularly
when in powder form, may be coated or uncoated; for example, it may
be at least partially coated with a surfactant to facilitate
processing. Suitable coating surfactants include, for example,
stearic acid, palmitic acid, a salt of stearate, a salt of
palmitate, and mixtures thereof. Other surfactants that also may be
utilized include lauric acid, oleic acid, capric acid, myristic
acid, linoleic acid, and mixtures thereof. Still other surfactants
that also may be utilized include polyethylene oxide, polyethylene
glycol, benzotriazole, poly(ethylene glycol)acetic acid, and other
similar organic molecules. Suitable counter-ions for use in a
coating surfactant include without limitation hydrogen, ammonium,
sodium, potassium, and mixtures thereof. When the electrically
conductive metal is silver, it may be coated, for example, with a
phosphorus-containing compound.
[0049] In an embodiment, one or more surfactants may be included in
the organic vehicle in addition to any surfactant included as a
coating of conductive metal powder used in the present paste
composition.
[0050] As further described below, the electrically conductive
metal can be dispersed in an organic vehicle that acts as a carrier
for the metal phase and other constituents present in the
formulation.
B. Alkaline-Earth-Metal Boron Bismuth Oxide
[0051] The present paste composition includes a fusible
alkaline-earth-metal boron bismuth oxide. The term "fusible," as
used herein, refers to the ability of a material to become fluid
upon heating, such as the heating employed in a firing operation.
In some embodiments, the fusible material is composed of one or
more fusible subcomponents. For example, the fusible material may
comprise a glass material, or a mixture of two or more glass
materials. Glass material in the form of a fine powder, e.g., as
the result of a comminution operation, is often termed "frit" and
is readily incorporated in the present paste composition.
[0052] As used herein, the term "glass" refers to a particulate
solid form, such as an oxide or oxyfluoride, that is at least
predominantly amorphous, meaning that short-range atomic order is
preserved in the immediate vicinity of any selected atom, that is,
in the first coordination shell, but dissipates at greater
atomic-level distances (i.e., there is no long-range periodic
order). Hence, the X-ray diffraction pattern of a fully amorphous
material exhibits broad, diffuse peaks, and not the well-defined,
narrow peaks of a crystalline material. In the latter, the regular
spacing of characteristic crystallographic planes give rise to the
narrow peaks, whose position in reciprocal space is in accordance
with Bragg's law. A glass material also does not show a substantial
crystallization exotherm upon heating close to or above its glass
transition temperature or softening point, T.sub.g, which is
defined as the second transition point seen in a differential
thermal analysis (DTA) scan. In an embodiment, the softening point
of glass material used in the present paste composition is in the
range of 300 to 800.degree. C.
[0053] It is also contemplated that some or all of the
alkaline-earth-metal boron bismuth oxide material may be composed
of material that exhibits some degree of crystallinity. For
example, in some embodiments, a plurality of oxides are melted
together and quenched as set forth above, resulting in a material
that is partially amorphous and partially crystalline. As would be
recognized by a skilled person, such a material would produce an
X-ray diffraction pattern having narrow, crystalline peaks
superimposed on a pattern with broad, diffuse peaks. Alternatively,
one or more constituents, or even substantially all of the fusible
material, may be predominantly or even substantially fully
crystalline. In an embodiment, crystalline material useful in the
fusible material of the present paste composition may have a
melting point of at most 800.degree. C.
[0054] The fusible material used in the present paste composition
is an alkaline-earth-metal boron bismuth oxide. As used herein, the
term "alkaline-earth-metal boron bismuth oxide" refers to an oxide
material containing alkaline-earth metal, boron, and bismuth
cations that together comprise at least 75% of the cations present
in the material, and wherein the minimum content of alkaline-earth
metal, boron, and bismuth cations is at least 10, 14, and 10 cation
%, respectively. In various embodiments, the combination of
alkaline-earth metal, boron, and bismuth cations represents at
least 75%, 80%, 90%, 95%, or up to 100% of the cations in the
alkaline-earth metal boron oxide. The alkaline-earth metals useful
in the present paste composition are Mg, Ca, Sr, Ba, and mixtures
thereof.
[0055] The alkaline-earth-metal boron bismuth oxide used in the
present paste composition is described herein as including
percentages of certain components. Specifically, the composition
may be specified by denominating individual components that may be
combined in the specified percentages to form a starting material
that subsequently is processed, e.g., as described herein, to form
a glass or other fusible material. Such nomenclature is
conventional to one of skill in the art. In other words, the
composition contains certain components, and the percentages of
those components may be expressed as weight percentages of the
corresponding oxide or other forms.
[0056] Alternatively, some of the compositions herein are set forth
by cation percentages, which are based on the total cations
contained in the alkaline-earth-metal boron bismuth oxide. Of
course, compositions thus specified include the oxygen or other
anions associated with the various cations. A skilled person would
recognize that compositions could equivalently be specified by
weight percentages of the constituents, and would be able to
perform the required numerical conversions. The skilled person
would also recognize that some of the cations incorporated in the
present composition can exist in different valences. Compounds used
to formulate the composition thus may have such cations in any
convenient valence.
[0057] The alkaline-earth-metal boron bismuth oxide included in the
present paste composition optionally incorporates other oxides,
including oxides of one or more of the elements Al, Li, Na, K, Rb,
Cs, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Si, Mo, W, Hf, Ag,
Ga, Ge, In, Sn, Sb, Se, Ru, P, Y, La and the other lanthanide
elements, and mixtures thereof. (The term "lanthanide elements" is
understood to include the chemical elements of the periodic table
having atomic numbers of 57 through 71, i.e., La-Lu.) This list is
meant to be illustrative, not limiting. In another embodiment, the
alkaline-earth-metal boron bismuth oxide further comprises an oxide
of one or more of the elements Li, P, Ti, Zn, Si, or Ag. The
foregoing substances are intimately mixed at an atomic level in the
alkaline-earth-metal boron bismuth oxide, e.g., by melting the
substances together. In some embodiments, the amount of these other
oxides incorporated is such that the total cation percentage of
them in the alkaline-earth-metal boron bismuth oxide is up to 5,
10, 20, or 25%.
[0058] Although oxygen is typically the predominant anion in the
alkaline-earth-metal boron bismuth oxide of the present paste
composition, some portion of the oxygen may be replaced by fluorine
or other halogen anions to alter certain properties, such as
chemical, thermal, or rheological properties of the oxide that
affect firing. In an embodiment, up to 10% of the oxygen anions of
the alkaline-earth-metal boron bismuth oxide in any of the
formulations of the present paste composition are replaced by one
or more halogen anions, including fluorine. For example, up to 10%
of the oxygen anions may be replaced by fluorine. Halogen anions
may be supplied from halides of any of the composition's cations,
including, but not limited to, NaCl, KBr, NaI, LiF, CaF.sub.2,
MgF.sub.2, BaCl.sub.2, and BiF.sub.3. In an embodiment, up to 10
anion percent of these oxygen anions can be substituted by
halogens, including fluorine. In another embodiment, the
replacement of oxygen may provide a content of up to 5 wt. % F, CI,
or Br.
[0059] For example, one of ordinary skill would recognize that
embodiments wherein the alkaline-earth-metal boron bismuth oxide
contains fluorine can be prepared using fluorine anions supplied
from a simple fluoride or an oxyfluoride. In an embodiment, the
desired fluorine content can be supplied by replacing some or all
of an oxide nominally incorporated in the composition with the
corresponding fluoride of the same cation, such as by replacing
some or all of the MgO, CaO, SrO, or BaO nominally included with
the amount of MgF.sub.2, CaF.sub.2, SrF.sub.2, or BaF.sub.2 needed
to attain the desired level of F content. Of course, the requisite
amount of F can be derived by replacing the oxides of more than one
cation of the alkaline-earth-metal boron bismuth oxide if desired.
Other fluoride sources could also be used, including sources such
as ammonium fluoride that would decompose during the heating in
typical glass preparation to leave behind residual fluoride anions.
Useful fluorides include, but are not limited to, CaF.sub.2,
BiF.sub.3, AlF.sub.3, NaF, LiF, ZrF.sub.4, TiF.sub.4, and
ZnF.sub.2.
[0060] The present paste composition may further comprise an
optional discrete oxide additive. It is contemplated that the
additive may comprise an oxide of one element, two or more discrete
oxides of various elements, or a discrete mixed oxide of multiple
elements. As used herein, the term "oxide of an element" includes
both the oxide compound itself and any other organic or inorganic
compound of the element, or the pure element itself if it oxidizes
or decomposes on heating to form the pertinent oxide. Such
compounds known to decompose upon heating include, but are not
limited to, carbonates, nitrates, nitrites, hydroxides, acetates,
formates, citrates, and soaps of the foregoing elements, and
mixtures thereof. For example, Zn metal, zinc acetate, zinc
carbonate, and zinc methoxide are potential additives that would
oxidize or decompose to form zinc oxide upon firing. The oxide
additive is discrete, in that it is not mixed at an atomic level
with the base alkaline-earth metal boron oxide, but is separately
present in the paste composition. In an embodiment, the discrete
oxide additive may be present in the paste composition in an amount
ranging from 0.01 to 5 wt. %, or 0.05 to 2.5 wt. %, or 0.1 to 1 wt.
%, based on the total weight of the paste composition.
[0061] Although in some embodiments the present composition
(including the fusible material contained therein) may contain a
small amount of lead, lead oxide, or other lead compound, e.g., in
an amount up to 5 cation % in the alkaline-earth-metal boron
bismuth oxide, other embodiments are lead-free. As used herein, the
term "lead-free paste composition" refers to a paste composition to
which no lead has been specifically added (either as elemental lead
or as a lead-containing alloy, compound, or other like substance),
and in which the amount of lead present as a trace component or
impurity is 1000 parts per million (ppm) or less. In some
embodiments, the amount of lead present as a trace component or
impurity is less than 500 ppm, or less than 300 ppm, or less than
100 ppm. Surprisingly and unexpectedly, photovoltaic cells
exhibiting desirable electrical properties, such as high conversion
efficiency, are obtained in some embodiments of the present
disclosure, notwithstanding previous belief in the art that
substantial amounts of lead must be included in a paste composition
to attain these levels.
[0062] Similarly, some embodiments of the present paste composition
comprise cadmium, e.g., in an amount up to 5 cation % in the
alkaline-earth-metal boron bismuth oxide, while others are
cadmium-free, again meaning that no Cd metal or compound is
specifically added and that the amount present as a trace impurity
is less than 1000 ppm, 500 ppm, 300 ppm, or 100 ppm.
[0063] In various embodiments, the alkaline-earth-metal boron
bismuth oxide of the present paste composition comprises, or
consists essentially of:
[0064] 10 to 40, or 10 to 35, or 12 to 30 cation % of an alkaline
earth metal selected from the group of Mg, Ca, Ba, Sr, and mixtures
thereof;
[0065] 14 to 65, or 25 to 65, or 30 to 60 cation % of B; and
[0066] 10 to 60, or 12 to 55, or 15 to 50 cation % of Bi.
[0067] In other embodiments, the alkaline-earth-metal boron bismuth
oxide of the present paste composition comprises, or consists
essentially of:
[0068] 10 to 40, or 10 to 35, or 12 to 30 cation % of alkaline
earth metal selected from the group of Mg, Ca, Ba, Sr, and mixtures
thereof;
[0069] 14 to 65, or 25 to 65, or 30 to 60 cation % of B;
[0070] 10 to 60, or 12 to 55, or 15 to 50 cation % of Bi;
[0071] 0 to 15, or 0 to 10, or 0 to 5 cation % of Li;
[0072] 0 to 15, or 0 to 10, or 0 to 5 cation % of Na;
[0073] 0 to 15, or 0 to 10, or 0 to 7 cation % of Si;
[0074] 0 to 15, or 0 to 10, or 0 to 8 cation % of P;
[0075] 0 to 20, or 0 to 15, or 0 to 10 cation % of Zn; and
[0076] 0 to 20, or 0 to 10, or 0 to 5 cation % of Ti,
[0077] plus incidental impurities.
[0078] One of ordinary skill in the art of glass chemistry would
further recognize that any of the foregoing alkaline-earth-metal
boron bismuth oxide material compositions, whether specified by
weight percentages or cation percentages of its constituent oxides,
may alternatively be prepared by supplying the required anions and
cations in requisite amounts from different components that, when
mixed and fired, yield the same overall composition. For example,
in various embodiments, phosphorus cations could be supplied either
from P.sub.2O.sub.5, or alternatively from a suitable organic or
inorganic phosphate that decomposes on heating to yield
P.sub.2O.sub.5, or from a metal phosphate in which the metal is
also a desired component of the final material. Lithium cations
could be supplied from Li.sub.2O, or alternatively from a suitable
organic or inorganic compound that decomposes on heating to yield
Li.sub.2O, such as lithium carbonate, lithium acetate, or lithium
hydroxide, or from a mixed metal oxide that includes lithium, such
as a lithium titanium oxide. It will be understood that the term
"mixed metal oxide" refers to an oxide comprising two or more
cations. Such mixed metal oxides include ones in which the cations
are located randomly or in defined positions of a crystallographic
structure. The skilled person would also recognize that depending
on the starting material employed, a certain portion of volatile
species, e.g., carbon dioxide, may be released during the process
of making a fusible material.
[0079] It is known to those skilled in the art that an
alkaline-earth-metal boron bismuth oxide such as one prepared by a
melting technique as described herein may be characterized by known
analytical methods that include, but are not limited to:
Inductively Coupled Plasma-Emission Spectroscopy (ICP-ES),
Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES),
and the like. In addition, the following exemplary techniques may
be used: X-Ray Fluorescence spectroscopy (XRF), Nuclear Magnetic
Resonance spectroscopy (NMR), Electron Paramagnetic Resonance
spectroscopy (EPR), Mossbauer spectroscopy, electron microprobe
Energy Dispersive Spectroscopy (EDS), electron microprobe
Wavelength Dispersive Spectroscopy (WDS), and Cathodoluminescence
(CL). A skilled person could calculate percentages of starting
components that could be processed to yield a particular fusible
material, based on results obtained with such analytical
methods.
[0080] The embodiments of the alkaline-earth-metal boron bismuth
oxide material described herein, including the compositions listed
in Tables I and IV, are not limiting; it is contemplated that one
of ordinary skill in the art of glass chemistry could make minor
substitutions of additional ingredients and not substantially
change the desired properties of the alkaline-earth-metal boron
bismuth oxide composition, including its interaction with a
substrate and any insulating layer thereon.
[0081] A median particle size of the alkaline-earth-metal boron
bismuth oxide material in the present composition may be in the
range of about 0.5 to 10 .mu.m, or about 0.8 to 5 .mu.m, or about 1
to 3 .mu.m, as measured using the Horiba LA-910 analyzer.
[0082] In an embodiment, the alkaline-earth-metal boron bismuth
oxide may be produced by conventional glass-making techniques and
equipment. For the examples provided herein, the ingredients were
weighed and mixed in the desired proportions and heated in a
platinum alloy crucible in a furnace. The ingredients may be heated
to a peak temperature (e.g., 800.degree. C. to 1400.degree. C., or
1000.degree. C. to 1200.degree. C.) and held for a time such that
the material forms a melt that is substantially liquid and
homogeneous (e.g., 20 minutes to 2 hours). The melt optionally is
stirred, either intermittently or continuously. In an embodiment,
the melting process results in a material wherein the constituent
chemical elements are fully mixed at an atomic level. The molten
material is then typically quenched in any suitable way including,
without limitation, passing it between counter-rotating stainless
steel rollers to form 0.25 to 0.50 mm thick platelets, by pouring
it onto a thick stainless steel plate, or by pouring it into water
or other quench fluid. The resulting particles are then milled to
form a powder or frit, which typically may have a d.sub.50 of 0.2
to 3.0 .mu.m.
[0083] Other production techniques may also be used for the present
alkaline-earth-metal boron bismuth oxide material. One skilled in
the art of producing such materials might therefore employ
alternative synthesis techniques including, but not limited to,
melting in non-precious metal crucibles, melting in ceramic
crucibles, sol-gel, spray pyrolysis, or others appropriate for
making powder forms of glass.
[0084] A skilled person would recognize that the choice of raw
materials could unintentionally include impurities that may be
incorporated into the alkaline-earth-metal boron bismuth oxide
material during processing. For example, these incidental
impurities may be present in the range of hundreds to thousands of
parts per million. Impurities commonly occurring in industrial
materials used herein are known to one of ordinary skill.
[0085] The presence of the impurities would not substantially alter
the properties of the alkaline-earth-metal boron bismuth oxide
itself, paste compositions made with the alkaline-earth-metal boron
bismuth oxide, or a fired device manufactured using the paste
composition. For example, a solar cell employing a conductive
structure made using the present paste composition may have the
efficiency described herein, even if the composition includes
impurities.
[0086] The alkaline-earth-metal boron bismuth oxide used in the
present composition is believed to assist in the partial or
complete penetration of the oxide or nitride insulating layer
commonly present on a silicon semiconductor wafer during firing. As
described herein, this at least partial penetration may facilitate
the formation of an effective, mechanically robust electrical
contact between a conductive structure manufactured using the
present composition and the underlying silicon semiconductor of a
photovoltaic device structure.
[0087] The alkaline-earth-metal boron bismuth oxide material in the
present paste composition may optionally comprise a plurality of
separate fusible substances, such as one or more frits, or a
substantially crystalline material with additional frit material.
In an embodiment, a first fusible subcomponent is chosen for its
capability to rapidly etch an insulating layer, such as that
typically present on the front surface of a photovoltaic cell;
further the first fusible subcomponent may have strong etching
power and low viscosity. A second fusible subcomponent is
optionally included to slowly blend with the first fusible
subcomponent to alter the chemical activity. Preferably, the
composition is such that the insulating layer is partially removed
but without attacking the underlying emitter diffused region, which
would shunt the device, were the corrosive action to proceed
unchecked. Such fusible materials may be characterized as having a
viscosity sufficiently high to provide a stable manufacturing
window to remove insulating layers without damage to the diffused
p-n junction region of a semiconductor substrate. Ideally, the
firing process results in a substantially complete removal of the
insulating layer without further combination with the underlying Si
substrate or the formation of substantial amounts of non-conducting
or poorly conducting inclusions.
C. Optional Oxide Additive
[0088] As noted above, an optional oxide may be included in the
present paste composition as a discrete additive, such as an oxide
of one or more of Al, Li, Na, K, Rb, Cs, Ti, V, Cr, Mn, Fe, Co, Ni,
Cu, Zn, Zr, Nb, Si, Mo, W, Hf, Ag, Ga, Ge, In, Sn, Sb, Se, Ru, Bi,
P, Y, La, or mixtures thereof, or a substance that forms such an
oxide upon heating. The oxide additive can be incorporated in the
paste composition in a powder form as received from the supplier,
or the powder can be ground or milled to a smaller average particle
size. Particles of any size can be employed, as long as they can be
incorporated into the present paste composition and provide its
required functionality. In an embodiment, the paste composition
comprises up to 5 wt. % of the discrete oxide additive.
[0089] Any size-reduction method known to those skilled in the art
can be employed to reduce particle size to a desired level. Such
processes include, without limitation, ball milling, media milling,
jet milling, vibratory milling, and the like, with or without a
solvent present. If a solvent is used, water is the preferred
solvent, but other solvents may be employed as well, such as
alcohols, ketones, and aromatics. Surfactants may be added to the
solvent to aid in the dispersion of the particles, if desired.
II. Organic Vehicle
[0090] The inorganic components of the present composition are
typically mixed with an organic vehicle to form a relatively
viscous material referred to as a "paste" or an "ink" that has a
consistency and rheology that render it suitable for printing
processes, including without limitation screen printing. The mixing
is typically done with a mechanical system, and the constituents
may be combined in any order, as long as they are uniformly
dispersed and the final formulation has characteristics such that
it can be successfully applied during end use.
[0091] A wide variety of inert materials can be admixed in an
organic medium in the present composition including, without
limitation, an inert, non-aqueous liquid that may or may not
contain thickeners, binders, or stabilizers. By "inert" is meant a
material that may be removed by a firing operation without leaving
any substantial residue and that has no other effects detrimental
to the paste or the final conductor line properties.
[0092] The proportions of organic vehicle and inorganic components
in the present paste composition can vary in accordance with the
method of applying the paste and the kind of organic vehicle used.
In an embodiment, the present paste composition typically contains
about 50 to 95 wt. %, 76 to 95 wt. %, or 85 to 95 wt. %, of the
inorganic components and about 5 to 50 wt. %, 5 to 24 wt. %, or 5
to 15 wt. %, of the organic vehicle.
[0093] The organic vehicle typically provides a medium in which the
inorganic components are dispersible with a good degree of
stability. In particular, the composition preferably has a
stability compatible not only with the requisite manufacturing,
shipping, and storage, but also with conditions encountered during
deposition, e.g., by a screen printing process. Ideally, the
rheological properties of the vehicle are such that it lends good
application properties to the composition, including stable and
uniform dispersion of solids, appropriate viscosity and thixotropy
for printing, appropriate wettability of the paste solids and the
substrate on which printing will occur, a rapid drying rate after
deposition, and stable firing properties.
[0094] Substances useful in the formulation of the organic vehicle
of the present paste composition include, without limitation, ones
disclosed in U.S. Pat. No. 7,494,607 and International Patent
Application Publication No. WO 2010/123967 A2, both of which are
incorporated herein in their entirety for all purposes, by
reference thereto. The disclosed substances include
ethylhydroxyethyl cellulose, wood rosin, mixtures of ethyl
cellulose and phenolic resins, cellulose acetate, cellulose acetate
butyrate, polymethacrylates of lower alcohols, monobutyl ether of
ethylene glycol, monoacetate ester alcohols, and terpenes such as
alpha- or beta-terpineol or mixtures thereof with other solvents
such as kerosene, dibutylphthalate, butyl carbitol, butyl carbitol
acetate, hexylene glycol, and high-boiling alcohols and alcohol
esters.
[0095] Solvents useful in the organic vehicle include, without
limitation, ester alcohols and terpenes such as alpha- or
beta-terpineol or mixtures thereof with other solvents such as
kerosene, dibutylphthalate, butyl carbitol, butyl carbitol acetate,
hexylene glycol, and high-boiling alcohols and alcohol esters. A
preferred ester alcohol is the monoisobutyrate of
2,2,4-trimethyl-1,3-pentanediol, which is available commercially
from Eastman Chemical (Kingsport, Tenn.) as TEXANOL.TM.. Some
embodiments may also incorporate volatile liquids in the organic
vehicle to promote rapid hardening after application on the
substrate. Various combinations of these and other solvents are
formulated to provide the desired viscosity and volatility.
[0096] In an embodiment, the organic vehicle may include one or
more components selected from the group consisting of:
bis(2-(2butoxyethoxy)ethyl) adipate, dibasic esters, octyl epoxy
tallate, isotetradecanol, and a pentaerythritol ester of
hydrogenated rosin. The paste compositions may also include
additional additives or components.
[0097] The dibasic ester useful in the present paste composition
may comprise one or more dimethyl esters selected from the group
consisting of dimethyl ester of adipic acid, dimethyl ester of
glutaric acid, and dimethyl ester of succinic acid. Various forms
of such materials containing different proportions of the dimethyl
esters are available under the DBE.RTM. trade name from Invista
(Wilmington, Del.). For the present paste composition, a preferred
version is sold as DBE-3 and is said by the manufacturer to contain
85 to 95 weight percent dimethyl adipate, 5 to 15 weight percent
dimethyl glutarate, and 0 to 1.0 weight percent dimethyl succinate
based on total weight of dibasic ester.
[0098] Further ingredients optionally may be incorporated in the
organic vehicle, such as thickeners, stabilizers, and/or other
common additives known to those skilled in the art. The organic
vehicle may be a solution of one or more polymers in a solvent.
Additionally, effective amounts of additives, such as surfactants
or wetting agents, may be a part of the organic vehicle. Such added
surfactant may be included in the organic vehicle in addition to
any surfactant included as a coating on the conductive metal powder
of the paste composition. Suitable wetting agents include phosphate
esters and soya lecithin. Both inorganic and organic thixotropes
may also be present.
[0099] Among the commonly used organic thixotropic agents are
hydrogenated castor oil and derivatives thereof, but other suitable
agents may be used instead of, or in addition to, these substances.
It is, of course, not always necessary to incorporate a thixotropic
agent since the solvent and resin properties coupled with the shear
thinning inherent in any suspension may alone be suitable in this
regard.
[0100] A polymer frequently used in printable conductive metal
pastes is ethyl cellulose. Other exemplary polymers that may be
used include ethylhydroxyethyl cellulose, wood rosin and
derivatives thereof, mixtures of ethyl cellulose and phenolic
resins, cellulose acetate, cellulose acetate butyrate,
poly(methacrylate)s of lower alcohols, and monoalkyl ethers of
ethylene glycol monoacetate.
[0101] Any of these polymers may be dissolved in a suitable
solvent, including those described herein.
[0102] The polymer in the organic vehicle may be present in the
range of 0.1 wt. % to 5 wt. % of the total composition. The present
paste composition may be adjusted to a predetermined,
screen-printable viscosity, e.g., with additional solvent(s).
III. Formation of Conductive Structures
[0103] An aspect of the invention provides a process that may be
used to form a conductive structure on a substrate. The process
generally comprises the steps of providing the substrate, applying
a paste composition, and firing the substrate. Ordinarily, the
substrate is planar and relatively thin, thus defining first and
second major surfaces on its opposite sides.
Application
[0104] The present composition can be applied as a paste onto a
preselected portion of a major surface of the substrate in a
variety of different configurations or patterns. The preselected
portion may comprise any fraction of the total first major surface
area, including substantially all of the area. In an embodiment,
the paste is applied on a semiconductor substrate, which may be
single-crystal, cast mono, multi-crystal, polycrystalline, or
ribbon silicon, or any other semiconductor material.
[0105] The application can be accomplished by a variety of
deposition processes, including printing. Exemplary deposition
processes include, without limitation, plating, extrusion or
co-extrusion, dispensing from a syringe, and screen, inkjet,
shaped, multiple, and ribbon printing. The paste composition
ordinarily is applied over any insulating layer present on the
first major surface of the substrate.
[0106] The conductive composition may be printed in any useful
pattern. For example, the electrode pattern used for the front side
of a photovoltaic cell commonly includes a plurality of narrow grid
lines or fingers connected to one or more bus bars. In an
embodiment, the width of the lines of the conductive fingers may be
20 to 200 .mu.m; 25 to 100 .mu.m; or 35 to 75 .mu.m. In an
embodiment, the thickness of the lines of the conductive fingers
may be 5 to 50 .mu.m; 10 to 35 .mu.m; or 15 to 30 .mu.m. Such a
pattern permits the generated current to be extracted without undue
resistive loss, while minimizing the area of the front side
obscured by the metallization, which reduces the amount of incoming
light energy that can be converted to electrical energy. Ideally,
the features of the electrode pattern should be well defined, with
a preselected thickness and shape, and have high electrical
conductivity and low contact resistance with the underlying
structure.
[0107] Conductors formed by printing and firing a paste such as
that provided herein are often denominated as "thick film"
conductors, since they are ordinarily substantially thicker than
traces formed by atomistic processes, such as those used in
fabricating integrated circuits. For example, thick film conductors
may have a thickness after firing of about 1 to 100 .mu.m.
Consequently, paste compositions that in their processed form
provide conductivity and are suitably applied using printing
processes are often called "thick film pastes" or "conductive
inks."
Firing
[0108] A firing operation may be used in the present process to
effect a substantially complete burnout of the organic vehicle from
the deposited paste. The firing typically involves volatilization
and/or pyrolysis of the organic materials. A drying operation
optionally precedes the firing operation, and is carried out at a
modest temperature to harden the paste composition by removing its
most volatile organics.
[0109] The firing process is believed to remove the organic
vehicle, sinter the conductive metal in the composition, and
establish electrical contact between the semiconductor substrate
and the fired conductive metal. Firing may be performed in an
atmosphere composed of air, nitrogen, an inert gas, or an
oxygen-containing mixture such as a mixed gas of oxygen and
nitrogen.
[0110] In one embodiment, the temperature for the firing may be in
the range between about 300.degree. C. to about 1000.degree. C., or
about 300.degree. C. to about 525.degree. C., or about 300.degree.
C. to about 650.degree. C., or about 650.degree. C. to about
1000.degree. C. The firing may be conducted using any suitable heat
source. In an embodiment, the firing is accomplished by passing the
substrate bearing the printed paste composition pattern through a
belt furnace at high transport rates, for example between about 100
to about 500 cm per minute, with resulting hold-up times between
about 0.05 to about 5 minutes. Multiple temperature zones may be
used to control the desired thermal profile, and the number of
zones may vary, for example, between 3 to 11 zones. The temperature
of a firing operation conducted using a belt furnace is
conventionally specified by the furnace set point in the hottest
zone of the furnace, but it is known that the peak temperature
attained by the passing substrate in such a process is somewhat
lower than the highest set point. Other batch and continuous rapid
fire furnace designs known to one of skill in the art are also
contemplated.
[0111] In a further embodiment, other conductive and device
enhancing materials are applied prior to firing to the opposite
type region of the semiconductor device. The various materials may
be applied and then co-fired, or they may be applied and fired
sequentially.
[0112] In an embodiment, the opposite type region may be on the
non-illuminated (back) side of the device, i.e., its second major
surface. The materials serve as electrical contacts, passivating
layers, and solderable tabbing areas. In an aspect of this
embodiment, the back-side conductive material may contain aluminum.
Exemplary back-side aluminum-containing compositions and methods of
application are described, for example, in US 2006/0272700, which
is hereby incorporated herein in its entirety for all purposes by
reference thereto. Suitable solderable tabbing materials include
those containing aluminum and silver. Exemplary tabbing
compositions containing aluminum and silver are described, for
example in US 2006/0231803, which is hereby incorporated herein in
its entirety for all purposes by reference thereto.
[0113] In a further embodiment, the present paste composition may
be employed in the construction of semiconductor devices wherein
the p and n regions are formed side-by-side in a substrate, instead
of being respectively adjacent to opposite major surfaces of the
substrate. In an implementation in this configuration, the
electrode-forming materials may be applied in different portions of
a single side of the substrate, e.g., on the non-illuminated (back)
side of the device, thereby maximizing the amount of light incident
on the illuminated (front) side.
Insulating Layer
[0114] In some embodiments of the invention, the paste composition
is used in conjunction with a substrate, such as a semiconductor
substrate, having an insulating layer present on one or more of the
substrate's major surfaces. The layer may comprise one or more
components selected from aluminum oxide, titanium oxide, silicon
nitride, SiN.sub.x:H (silicon nitride containing hydrogen for
passivation during subsequent firing processing), silicon oxide,
and silicon oxide/titanium oxide, and may be in the form of a
single, homogeneous layer or multiple sequential sub-layers of any
of these materials. Silicon nitride and SiN.sub.x:H are widely
used.
[0115] The insulating layer provides some embodiments of the cell
with an anti-reflective property, which lowers the cell's surface
reflectance of light incident thereon, thereby improving the cell's
utilization of the incident light and increasing the electrical
current it can generate. Thus, the insulating layer is often
denoted as an anti-reflective coating (ARC). The thickness of the
layer preferably is chosen to maximize the anti-reflective property
in accordance with the layer material's composition and refractive
index. In one approach, the deposition processing conditions are
adjusted to vary the stoichiometry of the layer, thereby altering
properties such as the refractive index to a desired value. For a
silicon nitride layer with a refractive index of about 1.9 to 2.0,
a thickness of about 700 to 900 .ANG. (70 to 90 nm) is
suitable.
[0116] The insulating layer may be deposited on the substrate by
methods known in the microelectronics art, such as any form of
chemical vapor deposition (CVD) including plasma-enhanced CVD
(PECVD) and thermal CVD, thermal oxidation, or sputtering. In
another embodiment, the substrate is coated with a liquid material
that under thermal treatment decomposes or reacts with the
substrate to form the insulating layer. In still another
embodiment, the substrate is thermally treated in the presence of
an oxygen- or nitrogen-containing atmosphere to form an insulating
layer. Alternatively, no insulating layer is specifically applied
to the substrate, but a naturally forming substance, such as
silicon oxide on a silicon wafer, may function as an insulating
layer.
[0117] The present method optionally includes the step of forming
the insulating layer on the semiconductor substrate prior to the
application of the paste composition.
[0118] In some implementations of the present process, the paste
composition is applied over any insulating layer present on the
substrate, whether specifically applied or naturally occurring. The
paste's fusible material and any additive present may act in
concert to combine with, dissolve, or otherwise penetrate some or
all of the thickness of any insulating layer material during
firing. Preferably, good electrical contact between the paste
composition and the underlying semiconductor substrate is thereby
established. Ideally, the firing results in a secure attachment of
the conductive metal structure to the substrate, with a
metallurgical bond being formed over substantially all the area of
the substrate covered by the conductive element. In an embodiment,
the conductive metal is separated from the silicon by a
nanometer-scale interfacial film layer (typically of order 5 nm or
less) through which the photoelectrons tunnel. In another
embodiment, contact is made between the conductive metal and the
silicon by a combination of direct metal-to-silicon contact and
tunneling through thin interfacial film layers.
[0119] Firing also promotes the formation of both good electrical
conductivity in the conductive element itself and a low-resistance
connection to the substrate, e.g., by sintering the conductive
metal particles and etching through the insulating layer. While
some embodiments may function with electrical contact that is
limited to conductive domains dispersed over the printed area, it
is preferred that the contact be uniform over substantially the
entire printed area.
Structures
[0120] An embodiment of the present invention relates to a
structure comprising a substrate and a conductive electrode, which
may be formed by the process described above.
Semiconductor Device Manufacture
[0121] The structures described herein may be useful in the
manufacture of semiconductor devices, including photovoltaic
devices. An embodiment of the invention relates to a semiconductor
device containing one or more structures described herein. Another
embodiment relates to a photovoltaic device containing one or more
structures described herein. Still further, there is provided a
photovoltaic cell containing one or more structures described
herein and a solar panel containing one or more of these
structures.
[0122] In another aspect, the present invention relates to a
device, such as an electrical, electronic, semiconductor,
photodiode, or photovoltaic device. Various embodiments of the
device include a junction-bearing semiconductor substrate and an
insulating layer, such as a silicon nitride layer, present on a
first major surface of the substrate.
[0123] One possible sequence of steps implementing the present
process for manufacture of a photovoltaic cell device is depicted
by FIGS. 1A-1F.
[0124] FIG. 1A shows a p-type substrate 10, which may be
single-crystal, multi-crystalline, or polycrystalline silicon. For
example, substrate 10 may be obtained by slicing a thin layer from
an ingot that has been formed from a pulling or casting process.
Surface damage and contamination (from slicing with a wire saw, for
example) may be removed by etching away about 10 to 20 .mu.m of the
substrate surface using an aqueous alkali solution such as aqueous
potassium hydroxide or aqueous sodium hydroxide, or using a mixture
of hydrofluoric acid and nitric acid. In addition, the substrate
may be washed with a mixture of hydrochloric acid and optional
hydrogen peroxide to remove heavy metals such as iron adhering to
the substrate surface. Substrate 10 may have a first major surface
12 that is textured to reduce light reflection. Texturing may be
produced by etching a major surface with an aqueous alkali solution
such as aqueous potassium hydroxide or aqueous sodium hydroxide.
Substrate 10 may also be formed from a silicon ribbon.
[0125] In FIG. 1B, an n-type diffusion layer 20 is formed to create
a p-n junction with p-type material below. The n-type diffusion
layer 20 can be formed by any suitable doping process, such as
thermal diffusion of phosphorus (P) provided from phosphorus
oxychloride (POCl.sub.3) or ion implantation. In the absence of any
particular modifications, the n-type diffusion layer 20 is formed
over the entire surface of the silicon p-type substrate. The depth
of the diffusion layer can be varied by controlling the diffusion
temperature and time, and is generally formed in a thickness range
of about 0.3 to 0.5 .mu.m. The n-type diffusion layer may have a
sheet resistivity from several tens of ohms per square up to about
120 ohms per square.
[0126] After protecting one surface of the n-type diffusion layer
20 with a resist or the like, the n-type diffusion layer 20 is
removed from most surfaces by etching so that it remains only on
the first major surface 12 of substrate 10, as shown in FIG. 1C.
The resist is then removed using an organic solvent or the
like.
[0127] Next, as shown in FIG. 1D, an insulating layer 30, which
also functions as an anti-reflective coating, is formed on the
n-type diffusion layer 20. The insulating layer is commonly silicon
nitride, but can also be a layer of another material, such as
SiN.sub.x:H (i.e., the insulating layer comprises hydrogen for
passivation during subsequent firing processing), titanium oxide,
silicon oxide, mixed silicon oxide/titanium oxide, or aluminum
oxide. The insulating layer can be in the form of a single layer or
multiple layers of the same or different materials.
[0128] Next, electrodes are formed on both major surfaces 12 and 14
of the substrate. As shown in FIG. 1E, a paste composition 500 of
this invention is screen printed on the insulating layer 30 of the
first major surface 12 and then dried. For a photovoltaic cell,
paste composition 500 is typically applied in a predetermined
pattern of conductive lines extending from one or more bus bars
that occupy a predetermined portion of the surface. In addition,
aluminum paste 60 and back-side silver paste 70 are screen printed
onto the back side (the second major surface 14 of the substrate)
and successively dried. The screen printing operations may be
carried out in any order. For the sake of production efficiency,
all these pastes are typically processed by co-firing them at a
temperature in the range of about 700.degree. C. to about
975.degree. C. for a period of from several seconds to several tens
of minutes in air or an oxygen-containing atmosphere. An
infrared-heated belt furnace is conveniently used for high
throughput.
[0129] As shown in FIG. 1F, the firing causes the depicted paste
composition 500 on the front side to sinter and penetrate through
the insulating layer 30, thereby achieving electrical contact with
the n-type diffusion layer 20, a condition known as "fire through."
This fired-through state, i.e., the extent to which the paste
reacts with and passes through the insulating layer 30, depends on
the quality and thickness of the insulating layer 30, the
composition of the paste, and on the firing conditions. A
high-quality fired-through state is believed to be an important
factor in obtaining high conversion efficiency in a photovoltaic
cell. Firing thus converts paste 500 into electrode 501, as shown
in FIG. 1F.
[0130] The firing further causes aluminum to diffuse from the
back-side aluminum paste into the silicon substrate, thereby
forming a p+ layer 40, containing a high concentration of aluminum
dopant. This layer is generally called the back surface field (BSF)
layer, and helps to improve the energy conversion efficiency of the
solar cell. Firing converts the dried aluminum paste 60 to an
aluminum back electrode 61. The back-side silver paste 70 is fired
at the same time, becoming a silver or silver/aluminum back
electrode 71. During firing, the boundary between the back-side
aluminum and the back-side silver assumes the state of an alloy,
thereby achieving electrical connection. Most areas of the back
electrode are occupied by the aluminum electrode, owing in part to
the need to form a p+ layer 40. Since there is no need for incoming
light to penetrate the back side, substantially the entire surface
may be covered. At the same time, because soldering to an aluminum
electrode is unfeasible, a silver or silver/aluminum back electrode
is formed on limited areas of the back side as an electrode to
permit soldered attachment of interconnecting copper ribbons or the
like.
[0131] While the present invention is not limited by any particular
theory of operation, it is believed that, upon firing, the
alkaline-earth-metal boron bismuth oxide material, with any
additive component present acting in concert, promotes rapid
etching of the insulating layer conventionally used on the front
side of a photovoltaic cell. Efficient etching in turn permits the
formation of a low resistance, front-side electrical contact
between the metal(s) of the composition and the underlying
substrate.
[0132] It will be understood that the present paste composition and
process may also be used to form electrodes, including a front-side
electrode, of a photovoltaic cell in which the p- and n-type layers
are reversed from the construction shown in FIGS. 1A-1F, so that
the substrate is n-type and a p-type material is formed on the
front side.
[0133] In yet another embodiment, this invention provides a
semiconductor device that comprises a semiconductor substrate
having a first major surface; an insulating layer optionally
present on the first major surface of the substrate; and, disposed
on the first major surface, a conductive electrode pattern having a
preselected configuration and formed by firing a paste composition
as described above.
[0134] A semiconductor device fabricated as described above may be
incorporated into a photovoltaic cell. In another embodiment, this
invention thus provides a photovoltaic cell array that includes a
plurality of the semiconductor devices as described, and made as
described, herein.
Lightly Doped Emitter (LDE) Wafers
[0135] The present paste composition is useful in constructing
photovoltaic cells using conventional, highly doped emitter (HDE)
wafers, as well as so-called "lightly doped emitter" (LDE)
wafers.
[0136] Si solar cells are made by adding controlled impurities
(called dopants) to purified Si. Different dopants impart positive
(p-type) and negative (n-type) semiconducting properties to the Si.
The boundary (junction) between the p-type and n-type Si has an
associated (built-in) voltage that provides power to electrical
charge carriers in the solar cell. Dopant concentration must be
controlled to achieve optimal cell performance. A high dopant
concentration in the emitter ordinarily imparts low electrical
emitter sheet resistivity and enables a low resistivity metal
contact to be made at the Si surface, thereby decreasing resistance
losses. However, a high dopant concentration also may introduce
crystalline defects or electrical perturbations in the Si lattice
that increase recombination losses.
[0137] A common Si solar cell design comprises a .about.200 micron
thick p-type Si wafer coated with a 0.4 micron thick n-type Si
layer. The p-type wafer is the base. The n-type layer is the
emitter. This configuration is typically made by either diffusion
or ion implantation of phosphorus (P) dopant into the Si wafer.
[0138] Typical highly doped Si emitters (HDE) have total
[P.sub.surface] ranging from 9 to 15.times.10.sup.20 atoms/cm.sup.3
and active [P.sub.surface] ranging from 3 to 4.times.10.sup.20
atoms/cm.sup.3. Lightly doped emitters have total [P.sub.surface]
ranging from 0.9 to 2.9.times.10.sup.20 atoms/cm.sup.3 and active
[P.sub.surface] ranging from 0.6 to 2.0.times.10.sup.20
atoms/cm.sup.3. P dopant in excess of the active concentration
(inactive P) leads to Shockley-Read-Hall (SRH) recombination energy
loss. Active P dopant above 1.times.10.sup.20 atoms/cm.sup.3 leads
to Auger recombination energy loss.
[0139] Total dopant concentration is typically measured using the
SIMS (secondary ion mass spectrometry) depth profiling method
[Diffusion in Silicon, S. W. Jones, IC Knowledge LLC 2008, pages
56-62; see page 61]. Active dopant concentration is often measured
using SRP (spreading resistance probing) or ECV (electrochemical
capacitance voltage) methods.
[0140] Solar cell embodiments employing lightly doped emitters in
some instances achieve improved solar cell performance by
decreasing the losses resulting from electron-hole recombination at
the front surface. However, the inherent potential of LDE-based
cells to provide improved cell performance often is not fully
realized in practice because of the greater difficulty of forming
the high-quality metal contacts needed to efficiently extract
current from the operating cell.
[0141] As a result, wafers used for commercial solar cells
heretofore have typically employed high [P.sub.surface] emitters,
as discussed above, which degrade short wavelength response (short
wavelengths having a very high absorption coefficient in silicon
and are absorbed very close to the surface) and result in lower
open-circuit voltage V.sub.oc and short-circuit current density
J.sub.sc. The high [P.sub.surface] emitters enable formation of low
contact resistivity metallization contacts, without which contact
is poor and cell performance is degraded.
[0142] Nevertheless, there remains an improvement in cell
performance potentially attainable with LDE-based cells. Such cells
would require a thick-film metallization paste that can reliably
contact lightly doped, low [P.sub.surface] emitters without
damaging the emitter layer surface, while still providing low
contact resistance. Ideally, such a paste would enable
screen-printed crystalline silicon solar cells to have reduced
saturation current density at the front surface (J.sub.0e) and
accompanying increased V.sub.oc and J.sub.sc, and therefore
improved solar cell performance. Other desirable characteristics of
a paste would include high bulk conductivity, the ability to form
narrow, high-aspect-ratio finger lines in a metallization pattern
to further reduce series resistance and minimize shading of
incident light by the electrodes, and good adherence to the
substrate.
EXAMPLES
[0143] The operation and effects of certain embodiments of the
present invention may be more fully appreciated from Examples 1-11
described below. The embodiments on which these examples are based
are representative only, and the selection of those embodiments to
illustrate aspects of the invention does not indicate that
materials, components, reactants, conditions, techniques, and/or
configurations not described in the examples are not suitable for
use herein, or that subject matter not described in the examples is
excluded from the scope of the appended claims and equivalents
thereof.
Examples 1a and 2a
Paste Preparation
[0144] In accordance with the present disclosure,
alkaline-earth-metal boron bismuth oxide materials as set forth in
Table I were prepared. The compositions were formulated by
combining requisite amounts of the compounds BaCO.sub.3,
CaCO.sub.3, Bi.sub.2O.sub.3, B.sub.2O.sub.3, and SiO.sub.2. The
amount of each compound was selected to provide in the combined
alkaline-earth-metal boron bismuth oxide the cation percentages
listed in Table I.
[0145] The various ingredients for each composition were intimately
mixed by melting them in a covered Pt crucible that was heated in
air from room temperature to 1100.degree. C. over a period of 1
hour, and held at that temperature for 30 minutes. Each melt was
separately poured onto the flat surface of a cylindrically-shaped
stainless steel block (8 cm high, 10 cm in diameter). The cooled
buttons were pulverized to a -100 mesh coarse powder.
[0146] Then the coarse powder was ball milled in a polyethylene
container with zirconia media and a suitable liquid, such as water,
isopropyl alcohol, or water containing 0.5 wt. % TRITON.TM. X-100
octylphenol ethoxylate surfactant (available from Dow Chemical
Company, Midland, Mich.) until the d.sub.50 was in the range of 0.5
to 2 .mu.m.
TABLE-US-00001 TABLE I Alkaline-earth-metal Boron Bismuth Oxide
Material Compositions cation % cation % cation % cation % cation %
Example # B Bi Si Ba Ca 1a 50.00 12.50 6.25 18.75 12.50 2a 53.33
26.67 6.67 6.67 6.67
[0147] In accordance with an aspect of the invention, the
alkaline-earth-metal boron bismuth oxide compositions of Examples
1a and 2a were combined with silver powder and an organic vehicle
to form paste compositions suitable for screen printing.
[0148] The silver powder used was represented by the manufacturer
as having a predominantly spherical shape. The powder was found to
have a particle size distribution with a d.sub.50 of about 2.3
.mu.m by measurement in an isopropyl alcohol dispersion using a
Horiba LA-910 analyzer.
[0149] The organic vehicle was prepared as a master batch using a
planetary, centrifugal Thinky mixer (available from Thinky USA,
Inc., Laguna Hills, Calif.) to mix the ingredients listed in Table
II below, with percentages given by weight.
TABLE-US-00002 TABLE II Organic Vehicle Composition Ingredient wt.
% 11% ethyl cellulose (50-52% ethoxyl) 13.98 dissolved in TEXANOL
.TM. solvent 8% ethyl cellulose (48-50% ethoxyl) 5.38 dissolved in
TEXANOL .TM. solvent tallowpropylenediamine dioleate 10.75
pentaerythritol ester of hydrogenated 26.88 rosin hydrogenated
castor oil derivative 5.38 dibasic ester 37.63
[0150] The paste compositions were formulated by combining
approximately 9.7 wt. % vehicle and 2, 3, or 4 wt. % of the
alkaline-earth-metal boron bismuth oxide materials of either
Example 1a or Example 2a, with the remainder being silver powder.
First, the milled oxide material and silver powder was combined in
a glass jar and tumble mixed for 15 minutes. This inorganic mixture
was then added by thirds to a Thinky jar containing the organic
ingredients and Thinky-mixed for 1 minute at 2000 RPM after each
addition. After the final addition, the paste was cooled and the
viscosity was adjusted to between about 300 and 400 Pa-s by adding
a suitable small portion of TEXANOL.TM. solvent and Thinky mixing
for 1 minute at 2000 RPM. Viscosities herein were measured with a
Brookfield viscometer (Brookfield Inc., Middleboro, Mass.) with a
#14 spindle and a #6 cup. Viscosity values were taken after 3
minutes at 10 RPM.
[0151] The paste was then milled on a three-roll mill (Charles Ross
and Son, Hauppauge, N.Y.) with a 25 .mu.m gap for 3 passes at zero
pressure and 3 passes at 100 psi (689 kPa). The paste was allowed
to sit overnight, and then its viscosity was adjusted to
approximately 300 Pa-s with small additions of solvent, if
necessary.
Examples 1b and 2b
Fabrication and Testing of Photovoltaic Cells
Cell Fabrication
[0152] Photovoltaic cells were fabricated in accordance with an
aspect of the invention using the paste compositions made with the
oxides of Examples 1a and 2a at different loadings to form the
front-side electrodes for the cells of Examples 1b and 2b. Examples
1b(1)-1b(3) and 2b(1)-2b(3) were made with different loadings of
the frits of Examples 1a and 2a, respectively. The amount of frit
in each composition is listed in Table III as a weight percentage
based on the total paste composition.
[0153] Conventional HDE Deutsche Cell multi-crystalline wafers
(.about.200 .mu.m thick, .about.65 ohms per square resistivity)
were used for fabrication and electrical testing. For convenience,
the experiments were carried out using 28 mm.times.28 mm "cut down"
wafers prepared by dicing 156 mm.times.156 mm starting wafers using
a diamond wafering saw. The test wafers were screen printed using
an AMI-Presco (AMI, North Branch, N.J.) MSP-485 screen printer,
first to form a full ground plane back-side conductor using a
conventional Al-containing paste, SOLAMET.RTM. PV381 (available
from DuPont, Wilmington, Del.), and thereafter to form a bus bar
and eleven conductor lines at a 0.254 cm pitch on the front surface
using the various exemplary paste compositions herein. After
printing and drying, cells were fired in a BTU rapid thermal
processing, multi-zone belt furnace (BTU International, North
Billerica, Mass.). Twenty five cells were printed using each paste;
5 cells were fired at each set point temperature in a 5-temperature
ladder ranging from set points 880 to 940.degree. C. After firing,
the median conductor line width was about 110 .mu.m and the mean
line height was about 15 .mu.m. The bus bar was 1.25 mm wide.
Performance of "cut-down" 28 mm.times.28 mm cells is known to be
impacted by edge effects which reduce the overall photovoltaic cell
efficiency by .about.5% from what would be obtained with full-size
wafers.
Electrical Testing
[0154] Electrical properties of photovoltaic cells as thus
fabricated were measured at 25.+-.1.0.degree. C. using an ST-1000
IV tester (Telecom STV Co., Moscow, Russia). The Xe arc lamp in the
IV tester simulated sunlight with a known intensity and irradiated
the front surface of the cell. The tester used a four contact
method to measure current (I) and voltage (V) at approximately 400
load resistance settings to determine the cell's I-V curve.
Efficiency, fill factor (FF), and series resistance (R.sub.a) were
obtained from the I-V curve for each cell. R.sub.a is defined in a
conventional manner as the negative of the reciprocal of the local
slope of the IV curve near the open circuit voltage. As recognized
by a person of ordinary skill, R.sub.a is conveniently determined
and a close approximation for R.sub.s, the true series resistance
of the cell. For each composition, an optimum firing temperature
was identified as the temperature that resulted in the highest
median efficiency, based on the 5-cell test group for each
composition and temperature. Electrical results for the cell groups
fired at the respective optimal firing temperature are depicted in
Table III below. Of course, this testing protocol is exemplary, and
other equipment and procedures for testing efficiencies will be
recognized by one of ordinary skill in the art.
TABLE-US-00003 TABLE III Electrical Properties of Multi-crystalline
Photovoltaic Cells wt. % frit Eff. FF Ra Example # in paste (%) (%)
(ohms) 1b(1) 2 13.96 66.3 0.3614 1b(2) 3 13.84 71.9 0.3104 1b(3) 4
13.77 67.9 0.3982 2b(1) 2 9.46 47.9 1.0443 2b(2) 3 12.80 62.6
0.4936 2b(3) 4 14.44 72.0 0.2992
Examples 3a to 11a
Paste Preparation
[0155] Using the same melting, quenching, and milling procedures
employed for Examples 1a and 2a, further alkaline-earth-metal boron
bismuth oxide materials in accordance with the present disclosure
were prepared, as set forth in Table IV.
TABLE-US-00004 TABLE IV Alkaline-earth-metal Boron Bismuth Oxide
Material Compositions Example cation % cation % cation % cation %
cation % cation % cation % cation % cation % # B Bi Ba Ca Li P Ti
Zn Si 3a 45.01 12.50 23.99 6.01 4.99 7.50 0.00 0.00 0.00 4a 44.98
12.50 20.00 5.01 5.01 7.50 5.01 0.00 0.00 5a 44.99 12.50 20.00 4.99
5.04 7.49 0.00 4.99 0.00 6a 50.00 12.50 18.75 12.50 0.00 0.00 0.00
0.00 6.25 7a 53.33 26.67 6.67 6.67 0.00 0.00 0.00 0.00 6.67 8a
32.00 40.00 18.00 5.00 5.00 0.00 0.00 0.00 0.00 9a 32.00 40.00 9.00
5.00 5.00 0.00 0.00 9.00 0.00 10a 14.40 57.60 18.00 5.00 5.00 0.00
0.00 0.00 0.00 11a 14.40 57.60 9.00 5.00 5.00 0.00 0.00 9.00
0.00
[0156] The alkaline-earth-metal boron bismuth oxide compositions of
Examples 3a to 11a were combined with the same silver powder and
organic vehicle and processed as described for Examples 1a and 2a
to form paste compositions suitable for screen printing.
Examples 3b to 11b
Fabrication and Testing of Photovoltaic Cells
[0157] Photovoltaic cells were fabricated and tested using the
techniques generally described above for Examples 1b and 2b. For
Examples 3b to 5b, paste compositions were prepared with 3.5 wt. %
loading of the frits of Examples 3a to 5a, respectively. For
Examples 6b to 11b, paste compositions were prepared at multiple
loadings of the frits of Examples 6a to 11a, respectively. Each
paste composition was then used to print front-side electrodes on
.about.200 .mu.m thick, .about.65 ohms per square resistivity, HDE
mono-crystalline wafer substrates (available from Gintech Energy
Corporation, Jhunan Township, Taiwan). Cells were again prepared
and tested on 28 mm.times.28 mm "cut down" wafers, using the same
firing and testing protocols as before.
[0158] Electrical properties obtained at the optimal firing
temperature for each composition are set forth in Table V. The data
demonstrate that operable photovoltaic cells can be fabricated with
the pastes of Examples 3a to 11a.
TABLE-US-00005 TABLE V Electrical Properties of Mono-crystalline
Photovoltaic Cells wt. % frit Eff. FF Ra Example # in paste (%) (%)
(ohms) 3b(1) 3.5 10.28 48.1 0.7289 4b(1) 3.5 13.38 60.8 0.5025
5b(1) 3.5 13.99 64.7 0.4193 6b(1) 2 13.96 66.3 0.3614 6b(2) 3 13.84
71.9 0.3104 6b(3) 4 13.77 67.9 0.3982 7b(1) 2 9.46 47.9 1.0443
7b(2) 3 12.80 62.6 0.4936 7b(3) 4 14.44 72.0 0.2992 8b(1) 2 14.49
64.3 0.3761 8b(2) 3 15.20 68.0 0.3272 9b(1) 2 15.19 66.8 0.3202
9b(2) 3 14.86 66.0 0.3919 10b(1) 2 15.28 67.6 0.3403 10b(2) 3 13.88
62.8 0.4320 11b(1) 2 15.29 69.4 0.3151 11b(2) 3 15.42 69.2
0.3255
[0159] Having thus described the invention in rather full detail,
it will be understood that this detail need not be strictly adhered
to but that further changes and modifications may suggest
themselves to one skilled in the art, all falling within the scope
of the invention as defined by the subjoined claims.
[0160] Where a range of numerical values is recited or established
herein, the range includes the endpoints thereof and all the
individual integers and fractions within the range, and also
includes each of the narrower ranges therein formed by all the
various possible combinations of those endpoints and internal
integers and fractions to form subgroups of the larger group of
values within the stated range to the same extent as if each of
those narrower ranges was explicitly recited. Where a range of
numerical values is stated herein as being greater than a stated
value, the range is nevertheless finite and is bounded on its upper
end by a value that is operable within the context of the invention
as described herein. Where a range of numerical values is stated
herein as being less than a stated value, the range is nevertheless
bounded on its lower end by a non-zero value.
[0161] In this specification, unless explicitly stated otherwise or
indicated to the contrary by the context of usage, where an
embodiment of the subject matter hereof is stated or described as
comprising, including, containing, having, being composed of, or
being constituted by or of certain features or elements, one or
more features or elements in addition to those explicitly stated or
described may be present in the embodiment. An alternative
embodiment of the subject matter hereof, however, may be stated or
described as consisting essentially of certain features or
elements, in which embodiment features or elements that would
materially alter the principle of operation or the distinguishing
characteristics of the embodiment are not present therein. A
further alternative embodiment of the subject matter hereof may be
stated or described as consisting of certain features or elements,
in which embodiment, or in insubstantial variations thereof, only
the features or elements specifically stated or described are
present. Additionally, the term "comprising" is intended to include
examples encompassed by the terms "consisting essentially of" and
"consisting of." Similarly, the term "consisting essentially of" is
intended to include examples encompassed by the term "consisting
of."
[0162] When an amount, concentration, or other value or parameter
is given as either a range, preferred range, or a list of upper
preferable values and lower preferable values, this is to be
understood as specifically disclosing all ranges formed from any
pair of any upper range limit or preferred value and any lower
range limit or preferred value, regardless of whether ranges are
separately disclosed. Where a range of numerical values is recited
herein, unless otherwise stated, the range is intended to include
the endpoints thereof, and all integers and fractions within the
range. It is not intended that the scope of the invention be
limited to the specific values recited when defining a range.
[0163] In this specification, unless explicitly stated otherwise or
indicated to the contrary by the context of usage,
[0164] (a) amounts, sizes, ranges, formulations, parameters, and
other quantities and characteristics recited herein, particularly
when modified by the term "about", may but need not be exact, and
may also be approximate and/or larger or smaller (as desired) than
stated, reflecting tolerances, conversion factors, rounding off,
measurement error, and the like, as well as the inclusion within a
stated value of those values outside it that have, within the
context of this invention, functional and/or operable equivalence
to the stated value; and
[0165] (b) all numerical quantities of parts, percentage, or ratio
are given as parts, percentage, or ratio by weight; the stated
parts, percentage, or ratio by weight may or may not add up to
100.
* * * * *